PURPOSE: To improve halftone reproducibility by binarizing inputted halftone image data with an error diffusion method, eliminating a high-pass spatial frequency component by applying a low-pass filter processing, and performing a binarization processing on it with a dither method.

CONSTITUTION: An error diffusion method binarization processing means 2 binarizes input multilevel image data 1 by using the error diffusion method. A low-pass filter processing means 4 eliminates the high-pass spatial frequency component from targeted binary image data 3 that is an intermediate result. A dither method binarization processing means 6 performs the binarization processing based on a dither pattern on multilevel image data 5. Therefore, when a dot image is inputted as an original image, the binarization processing by the error diffusion method is applied to the multilevel image data first. In such a way, no moire fringe is introduced, and dulling between thinning points can be remarkably reduced by suppressing the high-pass spatial frequency component with the low-pass filter processing and performing the binarization processing using the dither method finally.